Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for assessing a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. An IVUS device includes one or more ultrasound transducers arranged at a distal end of an elongate member. The elongate member is passed into the vessel thereby guiding the transducers to the area to be imaged. Once in place, the transducers emit ultrasonic energy in order to create an image of the vessel of interest. Ultrasonic waves are partially reflected by discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. Echoes from the reflected waves are received by the transducers and passed along to an IVUS imaging system. The imaging system processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the device is placed.
There are two general types of IVUS devices in use today: rotational and solid-state (also known as synthetic aperture phased array). For a typical rotational IVUS device, a single ultrasound transducer element is located at the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. In side-looking rotational devices, the transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the longitudinal axis of the device. In forward-looking rotational devices, the transducer element is pitched towards the distal tip so that the ultrasound beam propagates more towards the tip, in some devices, being emitted parallel to the longitudinal centerline. The fluid-filled sheath protects the vessel tissue from the spinning transducer and driveshaft while permitting ultrasound signals to propagate from the transducer into the tissue and back. As the driveshaft rotates, the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The same transducer then listens for the returning echoes reflected from various tissue structures. The IVUS imaging system assembles a two dimensional display of the tissue, vessel, heart structure, etc. from a sequence of pulse/acquisition cycles occurring during a single revolution of the transducer.
In contrast, solid-state IVUS devices utilize a scanner assembly that includes an array of ultrasound transducers connected to a set of transducer controllers. In side-looking and some forward-looking IVUS devices, the transducers are distributed around the circumference of the device. In other forward-looking IVUS devices, the transducers are a linear array arranged at the distal tip and pitched so that the ultrasound beam propagates closer to parallel with the longitudinal centerline. The transducer controllers select transducer sets for transmitting an ultrasound pulse and for receiving the echo signal. By stepping through a sequence of transmit-receive sets, the solid-state IVUS system can synthesize the effect of a mechanically scanned transducer element but without moving parts. Since there is no rotating mechanical element, the transducer array can be placed in direct contact with the blood and vessel tissue with minimal risk of vessel trauma. Furthermore, because there is no rotating element, the interface is simplified. The solid-state scanner can be wired directly to the imaging system with a simple electrical cable and a standard detachable electrical connector.
Owing to a variety of acoustic and device characteristics, both rotational and solid-state technologies are prone to artifacts and distortions that affect the resulting image. For example, the tendency of ultrasound pressure waves to radiate outward in many directions rather than being confined to a narrow beam may result in a transceiver detecting echoes from structures at oblique angles. Ultrasound transducers also tend to produce side lobes, secondary ultrasound pressure waves that may produce additional unwanted echo data. Synthetic aperture solid-state devices may also exhibit grating lobes caused by constructive and destructive interference from neighboring transducers. For non-sparse targets (e.g., tissue), main lobes, side lobes, and grating lobes all add upon each other in a complex acoustic interplay. The undesirable acoustic effects from these and other causes may reduce the contrast, clarity, and resolution of the resulting ultrasound image and may complicate the diagnostic process. Of course, these effects are not limited to intravascular ultrasound and occur in external ultrasound, transesophageal echo, and other ultrasound systems.
While existing ultrasound imaging systems have proved useful, there remains a need for improvements in the recognition and suppression of imaging artifacts. Doing so may reduce, clarify, or even eliminate the speckle noise that is characteristic of many solid-state designs. Even where noise is not completely eliminated, any clarification of clutter is often advantageous. In addition, artifact suppression may also reduce more subtle errors that cause a real structure to have an incorrect echo intensity. As echo intensity is important to determinations such as tissue characterization, tissue boundary/border detection, distance and/or area measurements, artifact suppression may noticeably improve diagnostic accuracy. Accordingly, the need exists for improved systems and techniques for identifying and removing ultrasound artifacts.